1. Trends in the evolution of mammals
Movement of limbs under body to support body above ground
Increased brain size
Increased efficiency of mastication
heterodont dentition
expanded size and mechanical advantages of jaw musculature
reduction and movement of “post-dentary bones” from jaw into middle ear

2. Cross-section through head of Fish

3. Cross-section through head of Tetrapod

4. Homologies of Postdentary Bones
Fishes Amniotes Mammals (among amniotes)
Hyomandibula Stapes (= “columella”) Stapes (=“stirup”)
Quadrate Quadrate Incus (=“anvil”)
Articular Articular Malleus (=“hammer”)
Angular Angular Tympanic (=“ectotympanic”
or “tympanic ring”)
Dentary Dentary Dentary
blue – middle ear ossicles of mammals
green – bones of mandible (= “lower jaw”)

5. Reduction of post-dentary bones in Synapsids

7. Mammalian Middle Ear Ossicles

8. Increased mechanical efficiency of jaw
use of enlarged braincase and temporal fenestra as areas of enlarged jaw muscle origin
larger lever arm
movement of mandibular articulation anteriorly
early synapsid late synapsid
arrow: vector of muscle contraction

10. Ophiacodon late Carboniferous-Permian Pelycosaur

11. Dimetrodon Permian Pelycosaur

12. Dimetrodon skull
synapsid temporal fenestra
mandibular articulation
homodont dentition

13. Inostranscevia Permian Cynodont

14. Lycaenops Permian Cynodont

15. Thrinaxodon Triassic cynodont
increased area for origin of masticatory muscles
ring-like shape of angular post-dentary bone
tendency towards heterodont dentition

16. Morganucodon Hadrocodium heterodont Triassic cynodont
Jurassic cynodont

17. Multituberculates Jurassic-early Cenozoic
small non-Therian mammals that superficially would have resembled rodents or insectivores
strange highly specialized heterodont teeth

18. Eomaia
oldest presumed Eutherian – lower Cretaceous of China

19. Late Cretaceous shrew-like Therians of Asia
Ukaatherium
Ukaatherium
Asioryctes

20. Archaic Paleogene Eutherian Mammals
e.g., Pantodonts
e.g., Condylarths

21. Archaic Paleogene Eutherian Mammals
e.g., Creodonts
e.g., Oreodonts

22. Early Paleogene Primates
Plesiadapis
Carpolestes